This invention relates to ballasts for discharge lamps operated from alternating current line sources. More particularly, the present invention relates to a resistive lamp ballast operating together with means for sensing lamp voltage and means for applying the output of an oscillator circuit to the lamp to effect lamp re-ignition.
For proper operation, discharge lamps require a current limiting or regulating device electrically connected between the lamp and the electrical energy source for the lamp. The most ubiquitous and available source of lamp energy is the conventional 50 or 60 cycle alternating current voltage present at conventional electrical outlets. Discharge lamps conventionally "see" this electrical power source as a source of constant voltage. Moreover, once lit, the resistance exhibited by a discharge lamp falls off with increasing lamp current. Thus, such lamps are not self-ballasting like conventional incandescent lamps whose resistance increases with filament current. Accordingly, for discharge lamps a current limiting device is required. This ballasting device may be an inductor, capacitor, resistor or an appropriately designed electronic circuit. However, the resistive ballast has a significant advantage of having a very low cost and is, accordingly, desirable for many discharge lamp applications.
Furthermore, because of the wide-spread availability of 50 or 60 Hz alternating current power, it is, accordingly, desirable to be able to operate discharge lamps directly from such an electrical energy source. However, when operated on alternating current directly, discharge lamps extinguish twice each cycle at those points in time at which the lamp current goes to zero. Inductive ballasts aid in the re-ignition of discharge lamps operated under alternating current conditions, since they provide a voltage spike each time the lamp current drops to zero. This voltage is phased so that it adds to the line voltage. However, resistive ballasts are generally considered undesirable for alternating current operation since they cannot provide voltage spikes which may be employed for lamp re-ignition. Therefore, in resistive ballast circuits operating from alternating current lines, lamp re-ignition voltage is limited by the available line voltage. However, the re-ignition voltage of certain lamps, such as miniature arc lamps, may be higher than the available line voltage. One such miniature arc discharge lamp is the Halarc lamp which is currently preferably operated from a direct current power supply since this eliminates the need to re-ignite the lamp twice during each alternating current cycle. However, direct current operation in such lamps can create cataphoretic effects which may have an adverse effect on lamp color as a function of lamp operating position. Therefore, it is also desirable to be able to operate this lamp and other discharge lamps directly from alternating current sources. Additionally, as discussed above, it is also desirable to operate such lamps using a resistive ballast. However, in such situations, means must be provided to ensure lamp re-ignition during conditions of zero current.